Fix for “edit layer in external app” option

Hi, I have modified the externalapp.py file so that mypaint opens GImp3 and Inkscape for vector layers, I hope this can help others so they can use this feature that was not currently available.

You simply need to replace the file “externalapp.py” which is in “C:\Program Files\mypaint\mingw64\lib\mypaint\gui”